Just reporting in case someone else has this happen to them but probably just going to be a one-off
Long story short - my big missile command AI smart bomb just stopped moving after its initial show
Eventually - 18 hours later this morning - found it was failing when entering a repeat until loop with 2 or'd conditions and it turned out it was the rgba test one that caused the issue.
At this point in debugging, it occurred to me to try project on another computer - and it worked fine so I closed and restart my browser on main machine and it all started working again
Like I say, not seen/had this sort of difficult to debug issue before - Snap6 has been VERY solid with me until this point and hopefully it was just cosmic rays altering one bit in my RAM chips!
But putting it out just in case its not just a one-off